Signal processing circuit for a capacitive touch panel capable of switching between a differential-input sensor circuit and a single-input sensor circuit

ABSTRACT

This invention offers a signal processing circuit of an electrostatic capacity type touch panel which is capable of switching between a differential input mode and a single input mode and has an extended adjustable range of an offset in the single input mode. The signal processing circuit of this invention includes a first sensor circuit of a differential input type, a second sensor circuit of a single input type, a third and fourth electrostatic capacitors that are variable capacitors for calibration to adjust the offset in an output voltage of the first sensor circuit, and a switching control circuit to control so as to put in operation one of the first and second sensor circuits. The switching control circuit also controls so that the third and fourth electrostatic capacitors for calibration are connected in parallel to each other when the second sensor circuit is put in operation.

CROSS-REFERENCE OF THE INVENTION

This application claims priority from Japanese Patent Application No.2009-267415, the content of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a signal processing circuit of anelectrostatic capacity type touch panel.

2. Description of the Related Art

The electrostatic capacity type touch sensor has been known as a datainput device to various kinds of electronic devices such as a mobilephone, a portable audio device, a portable game console, a televisionand a personal computer.

A conventional signal processing circuit of an electrostatic capacitytype touch panel is explained referring to FIG. 10 and FIG. 11. A senseline 61 (touch pad) that has an electrostatic capacitor 62 having acapacitance C is disposed on a touch panel 60, as shown in FIG. 10.

The sense line 61 is connected to a non-inverting input terminal (+) ofa differential amplifier 63 (comparator) through a wiring 64. Areference voltage Vref is applied to an inverting input terminal (−) ofthe differential amplifier 63. A constant current power supply 65 isconnected to the wiring 64 that connects between the sense line 61 andthe non-inverting input terminal (+) of the differential amplifier 63.

Operations of the signal processing circuit of the electrostaticcapacity type touch panel are explained referring to FIG. 11. When afinger 66 of an operator is far away from the sense line 61, acapacitance associated with the sense line 61 is C. In this case, avoltage at the sense line 61 increases from 0 V in a reset state as theelectrostatic capacitor 62 connected with the sense line 61 is chargedby a constant current from the constant current power supply 65. Anoutput voltage of the differential amplifier 63 is inverted when thevoltage at the sense line 61 reaches the reference voltage Vref. Alength of time from the reset state to the inversion of the differentialamplifier 63 in this case is referred to as t1.

When the finger 66 of the operator approaches the sense line 61, on theother hand, the capacitance associated with the sense line 61 increasesto C+C′. The increment C′ is a capacitance of a capacitor formed betweenthe finger 66 of the operator and the sense line 61. As a result, thelength of time that the voltage at the sense line 61 takes from 0 V tothe reference voltage Vref increases to t2 (t2>t1). That is, it ispossible to detect whether the finger 66 of the operator has touched thesense line 61 or not, based on a difference (t2−t1) in the length oftime taken by the transition from the reset state to the inversion ofthe differential amplifier 63.

Technologies mentioned above are disclosed in Japanese PatentApplication Publication No. 2005-190950, for example.

However, the signal processing circuit described above is a single inputtype in which a signal from the single sense line 61 is inputted to thedifferential amplifier 63, and has a problem that the voltage at thesense line 61 is varied to cause malfunctioning when a noise is appliedto the sense line 61.

On the other hand, a differential input type signal processing circuitin which a difference between capacitances associated with two senselines is detected by an electric charge amplifier is tolerant of thenoise and is capable of forming a high sensitivity touch sensor. Thedifferential input type signal processing circuit is suitable for asingle-touch, which means that only one sense line is touched, but has aproblem that a touch position can be not detected in the case of amulti-touch, which means that two or more than two sense lines aresimultaneously touched. The problem is caused because the differencebetween the capacitances of the two sense lines is lost.

SUMMARY OF THE INVENTION

This invention provides a signal processing circuit of an electrostaticcapacity type touch panel that has a plurality of sense lines includingfirst and second sense lines and a drive line to which an alternatingcurrent drive signal is applied. The signal processing circuit includesa first sensor circuit of a differential input type which selects thefirst and second sense lines out of the plurality of sense lines anddetects a difference between a capacitance of a first electrostaticcapacitor formed between the first sense line and the drive line and acapacitance of a second electrostatic capacitor formed between thesecond sense line and the drive line, a second sensor circuit of asingle input type which selects the first sense line out of theplurality of sense lines and detects a change in the capacitance of thefirst electrostatic capacitor formed between the first sense line andthe drive line, first and second variable capacitors for calibrationadjusting an offset in an output voltage of the first sensor circuit,and a switching control circuit to control the first and second sensorcircuits so that either the first sensor circuit or the second sensorcircuit is put in operation and to control the first and second variablecapacitors for calibration so that the first and second variablecapacitors are connected in parallel to each other when the secondsensor is put in operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a touch sensor including an electrostaticcapacity type touch panel and signal processing circuits.

FIG. 2 shows a structure of a signal processing circuit of anelectrostatic capacity type touch panel according to an embodiment ofthis invention.

FIG. 3 shows a structure of a first sensor circuit of a differentialinput type.

FIG. 4 shows a structure of a second sensor circuit of a single inputtype.

FIG. 5 shows a structure of a variable capacitor.

FIGS. 6A and 6B are to explain operations of the first sensor circuit ofthe differential input type.

FIG. 7 shows waveforms of outputs of the first sensor circuit of thedifferential input type.

FIG. 8 is an operation timing chart of the signal processing circuit ofthe electrostatic capacity type touch panel according to the embodimentof this invention.

FIGS. 9A and 9B are to explain operations of the second sensor circuitof the single input type.

FIG. 10 shows a structure of a conventional signal processing circuit ofan electrostatic capacity type touch panel.

FIG. 11 is to explain operations of the conventional signal processingcircuit of the electrostatic capacity type touch panel.

FIG. 12 shows a structure of the second sensor circuit of the singleinput type.

FIG. 13 shows a structure of the second sensor circuit of the singleinput type.

FIG. 14 shows a structure of the first sensor circuit of thedifferential input type.

DETAILED DESCRIPTION OF THE INVENTION

A signal processing circuit of an electrostatic capacity type touchpanel according to an embodiment of this invention is hereafterdescribed referring to the drawings. An electrostatic capacity typetouch sensor 100 is formed to include a touch panel 1, signal processingcircuits 2X and 2Y and a microcomputer 3, as shown in FIG. 1. The signalprocessing circuits 2X and 2Y can be implemented with a single chip.

The touch panel 1 has X sense lines XL1-XL4 and an X drive line DRXLextending in an X direction on a glass substrate 200. The X drive lineDRXL is disposed on both sides of each of the X sense lines XL1-XL4. Thetouch panel 1 also has Y sense lines YL1-YL4 and a Y drive line DRYLextending in a Y direction on the glass substrate 200 and intersectingthe X sense lines XL1-XL4. The Y drive line DRYL is disposed on bothsides of each of the Y sense lines YL1-YL4. The X sense lines XL1-XL4,the X drive line DRXL, the Y sense lines YL1-YL4 and the Y drive lineDRYL are electrically insulated from each other with a dielectric layeror the like.

The signal processing circuits 2X and 2Y are disposed adjacent the touchpanel 1 on the glass substrate 200. It is preferable that the signalprocessing circuits 2X and 2Y are implemented with an LSI chip or formedon the glass substrate 200 using a thin film transistor (TFT) process.

The signal processing circuit 2X has first through fourth inputterminals CIN1-CIN4 and a drive terminal CDRV outputting a alternatingcurrent drive signal SCDRV (amplitude voltage Vref). The first inputterminal CIN1 is connected to the X sense line XL1, the second inputterminal CIN2 is connected to the X sense line XL3, the third inputterminal CIN3 is connected to the X sense line XL2 and the fourth inputterminal CIN4 is connected to the X sense line XL4. The drive terminalCDRV is connected to the X drive line DRXL.

Similarly, the signal processing circuit 2Y has first through fourthinput terminals CIN1-CIN4 and a drive terminal CDRV outputting aalternating current drive signal SCDRV (amplitude voltage Vref). Thefirst input terminal CIN1 is connected to the Y sense line YL1, thesecond input terminal CIN2 is connected to the Y sense line YL3, thethird input terminal CIN3 is connected to the Y sense line YL2 and thefourth input terminal CIN4 is connected to the Y sense line YL4. Thedrive terminal CDRV is connected to the Y drive line DRYL.

In addition, each of the signal processing circuits 2X and 2Y has aserial clock terminal SCL and a serial data terminal SDA. Both of theserial clock terminals SCL are connected to a serial clock line 4, whileboth of the serial data terminals SDA are connected to a serial dataline 5. In this case, the serial clock line 4 and the serial data line 5constitute an I²C (Inter-Integrated Circuit) bus.

A microcomputer 3, that is a master device, is provided on a PCBsubstrate (not shown) outside the glass substrate 200. The serial clockline 4 and the serial data line 5 are connected to the microcomputer 3through an FPC (Flexible Printed Circuit) or the like. With thestructure described above, it is made possible that data communicationis performed between the microcomputer 3 and the signal processingcircuits 2X and 2Y. Although the microcomputer is used as the masterdevice in the example described above, device other than themicrocomputer such as a DSP (Digital Signal Processor) or a logiccircuit may be used. Also, although the I²C bus is used in the serialcommunication in the example described above, other serial communicationsuch as an SPI (Serial Peripheral Interface) or UART (UniversalAsynchronous Receiver Transmitter) may be used.

Although the X sense lines XL1-XL4 and the Y sense lines YL1-YL4 aremade of four each of sense lines that make the minimum size of the touchpanel 1, the number of sense lines may be increased as required. In thatcase, each of the signal processing circuits 2X and 2Y is to beexpanded. Or the number of the sense lines is to be increased when thesignal processing circuits are implemented with a single chip.

A detailed structure of each of the signal processing circuits 2X and 2Yof the electrostatic capacity type touch panel is hereafter describedreferring to FIG. 2. Since the structure of the signal processingcircuit 2X is identical to the structure of the signal processingcircuit 2Y, it is good enough to describe only the structure of thesignal processing circuit 2Y.

The signal processing circuit 2Y is formed to include a selectioncircuit 10, a switching control circuit 11, a drive circuit 12 thatgenerates the alternating current drive signal SCDRV, an inverter 13, athird electrostatic capacitor C3, a fourth electrostatic capacitor C4, adifferential amplifier 14, a first feedback capacitor 15, a secondfeedback capacitor 16, an A/D converter 17, an I²C bus interface circuit18, a calibration circuit 19, an EEPROM 20, switches SW1-SW6 and areference voltage source 21 that generates a reference voltage ½ Vrefthat is a half of an amplitude voltage Vref of the alternating currentdrive signal SCDRV.

The signal processing circuit 2Y has a differential input mode and asingle input mode. Switching between the differential input mode and thesingle input mode is performed by turning the switches SW1-SW4 on/offwith the switching control circuit 11 and by switching selectingoperation with the selection circuit 10.

The switch SW1 is connected between the fourth electrostatic capacitorC4 and a wiring 22 that connects between a first output of the selectioncircuit 10 and a non-inverting input terminal (+) of the differentialamplifier 14. The switch SW2 is connected between the fourthelectrostatic capacitor C4 and a wiring 23 that connects between asecond output of the selection circuit 10 and an inverting inputterminal (−) of the differential amplifier 14.

The switch SW3 is connected between one end of the third electrostaticcapacitor C3 and one end of the fourth electrostatic capacitor C4 sothat the third electrostatic capacitor C3 and the fourth electrostaticcapacitor C4 are connected in parallel. The switch SW4 is connectedbetween the reference voltage source 21 and the inverting input terminal(−) of the differential amplifier 14 so that the reference voltage ½Vref is selectively applied to the inverting input terminal (−) of thedifferential amplifier 14. It is preferable that the switches SW1-SW4are made of CMOS analog switches.

Table 1 shows on/off positions of the switches SW1-SW4 in thedifferential input mode and the single input mode.

TABLE 1 Mode Differential Single Switch Input Mode Input Mode SW1 OFF ONSW2 ON OFF SW3 OFF ON or OFF SW4 OFF ON

(a) In the differential input mode, the switch SW1 is turned off, theswitch SW2 is turned on, the switch SW3 is turned off and the switch SW4is turned off. The selection circuit 10 has a first phase and a secondphase. In the first phase, it selects signals from the first inputterminal CIN1 and the second input terminal CIN2. That is, the firstinput terminal CIN1 is connected to the non-inverting input terminal (+)of the differential amplifier 14 through the wiring 22, and the secondinput terminal CIN2 is connected to the inverting input terminal (−) ofthe differential amplifier 14 through the wiring 23.

The selection circuit 10 selects signals from the third input terminalCIN3 and the fourth input terminal CIN4 in the second phase. That is,the third input terminal CIN3 is connected to the non-inverting inputterminal (+) of the differential amplifier 14 through the wiring 22, andthe fourth input terminal CIN4 is connected to the inverting inputterminal (−) of the differential amplifier 14 through the wiring 23.

As a result, there is formed a first sensor circuit of a differentialinput type, as shown in FIG. 3. FIG. 3 shows a case of the first phasein which the selection circuit 10 selects the signals from the firstinput terminal CIN1 and the second input terminal CIN2. In this case, afirst electrostatic capacitor C1 is formed between the Y sense line YL1connected to the first input terminal CIN1 and the Y drive line DRYL,while a second electrostatic capacitor C2 is formed between the Y senseline YL3 connected to the second input terminal CIN2 and the Y driveline DRYL, as shown in FIG. 1.

Then, the first electrostatic capacitor C1 is connected in series withthe third electrostatic capacitor C3, while the second electrostaticcapacitor C2 is connected in series with the fourth electrostaticcapacitor C4, as shown in FIG. 3. The alternating current drive signalSCDRV from the drive circuit 12 is applied to a common connection nodeof the first electrostatic capacitor C1 and the second electrostaticcapacitor C2, that is, the Y drive line DRYL. Also, an invertedalternating current drive signal *SCDRV, that is generated by invertingthe alternating current drive signal SCDRV from the drive circuit 12with the inverter 13, is applied to a common connecting node of thethird electrostatic capacitor C3 and the fourth electrostatic capacitorC4.

A connecting node N2 between the first electrostatic capacitor C1 andthe third electrostatic capacitor C3 is connected to the non-invertinginput terminal (+) of the differential amplifier 14. A connecting nodeN1 between the second electrostatic capacitor C2 and the fourthelectrostatic capacitor C4 is connected to the inverting input terminal(−) of the differential amplifier 14.

The first feedback capacitor 15 and the switch SW5 are connected betweenan inverting output terminal (−) and the non-inverting input terminal(+) of the differential amplifier 14, while the second feedbackcapacitor 16 and the switch SW6 are connected between a non-invertingoutput terminal (+) and the inverting input terminal (−) of thedifferential amplifier 14.

It is preferable that the switches SW5 and SW6 are made of CMOS analogswitches in order to have good linearity in signal transfercharacteristics. Also, it is preferable that the first and secondfeedback capacitors (Cf) 15 and 16 have the same capacitance CAf.

The first sensor circuit of the differential input type outputs anoutput voltage Vout that corresponds to a difference between acapacitance CA1 of the first electrostatic capacitor C1 and acapacitance CA2 of the second electrostatic capacitor C2. Its detailedoperation is to be described.

(b) In the single input mode, the switch SW1 is turned on, the switchSW2 is turned off, the switch SW3 is turned on and the switch SW4 isturned on. The selection circuit 10 sequentially selects each of thesignals from the first input terminal CIN1, the third input terminalCIN3, the second input terminal CIN2 and the fourth input terminal CIN4one after another, and applies the selected signal to the non-invertinginput terminal (+) of the differential amplifier 14 through the wiring22 as a first output.

As a result, there is formed a second sensor circuit of a single inputtype, as shown in FIG. 4. FIG. 4 shows a case in which the selectioncircuit 10 selects the signal from the first input terminal CIN1. Inthis case, the first electrostatic capacitor C1 is formed between the Ysense line YL1 connected to the first input terminal CIN1 and the Ydrive line DRYL, as shown in FIG. 1.

Then, the first electrostatic capacitor C1 is connected in series withthe third electrostatic capacitor C3, as shown in FIG. 4. The fourthelectrostatic capacitor C4 is not connected in series with the firstelectrostatic capacitor C1 when the switch SW3 is turned off. On theother hand, the fourth electrostatic capacitor C4 is connected in serieswith the first electrostatic capacitor C1 when the switch SW3 is turnedon. That is, the third electrostatic capacitor C3 and the fourthelectrostatic capacitor C4 are connected in parallel to each other, anda compound electrostatic capacitor C5 composed of them is connected inseries with the first electrostatic capacitor C1.

The alternating current drive signal SCDRV from the drive circuit 12 isapplied to one end of the first electrostatic capacitor C1, which is theY drive line DRYL in this case. Also, the inverted alternating currentdrive signal *SCDRV, that is generated by inverting the alternatingcurrent drive signal SCDRV from the drive circuit 12 with the inverter13, is applied to the common connecting node of the third electrostaticcapacitor C3 and the fourth electrostatic capacitor C4.

The connecting node N2 between the first electrostatic capacitor C1 andthe third electrostatic capacitor C3 is connected to the non-invertinginput terminal (+) of the differential amplifier 14. The referencevoltage ½ Vref from the reference voltage source 21 is applied to theinverting input terminal (−) of the differential amplifier 14.

The second sensor circuit of the single input type outputs an outputvoltage Vout that corresponds to a difference between the capacitanceCA1 of the first electrostatic capacitor C1 and a capacitance CA3 of thethird electrostatic capacitor C3 when the switch SW3 is turned off, andoutputs an output voltage Vout that corresponds to a difference betweenthe capacitance CA1 of the first capacitor C1 and a capacitance CA5 ofthe compound electrostatic capacitor C5 when the switch SW3 is turnedon. Its detailed operation is to be described.

Since the third electrostatic capacitor C3 or both the thirdelectrostatic capacitor C3 and the fourth electrostatic capacitor C4 inthe first sensor circuit of the differential input type is used as areference capacitor in the second sensor circuit of the single inputtype, an increase in the number of capacitors can be suppressed in thecase where the single input type and the differential input type areswitched.

Because the output voltage Vout of the first and second sensor circuitsis an analog signal, it can be not digital-signal-processed as it is.Thus, the output voltage Vout is converted into a digital signal withthe A/D converter 17. An output of the A/D converter 17 is convertedinto serial data in a predetermined format by the I²C bus interfacecircuit 18, and transmitted to the microcomputer 3 through the serialclock terminal SCL and the serial data terminal SDA. The microcomputer 3processes the serial data it received, and determines a touch positionon the touch panel 1.

Calibration performed by the first and second sensor circuits describedabove is explained referring to FIG. 2 and FIG. 5.

There is caused an offset in the output voltage Vout of the first sensorcircuit of the differential input type, when there is an imbalancebetween the capacitance CA1 of the first electrostatic capacitor C1 andthe capacitance CA2 of the second electrostatic capacitor C2, that is,when there is a difference between them, in an initial state (a state inwhich a finger of an operator or the like is too far away to bedetected). When the offset is caused, detection accuracy of the touchsensor is degraded. The offset can be adjusted when the third and fourthelectrostatic capacitors C3 and C4 are formed of variable capacitors.

That is, the calibration circuit 19 adjusts the capacitances CA3 and CA4of the third and fourth electrostatic capacitors C3 and C4 so that theoffset becomes a desired value, which is preferably a minimum value,based on the output voltages Vout (preferably digital values after theA/D conversion) of the first and second sensor circuits in the initialstate, as shown in FIG. 2.

As for the calibration of the first sensor circuit of the differentialinput type (Refer to FIG. 3.), it is preferable that the capacitancesCA1-CA4 of the first through fourth electrostatic capacitors C1-C4 areequal to each other (CA1=CA2=CA3=CA4) in the initial state. However,there is caused the offset in the output voltage Vout when thecapacitance CA1 of the first electrostatic capacitor C1 is larger thanthe capacitance CA2 of the second electrostatic capacitor C2 by ΔC(CA1=C+ΔC, CA2=C) due to variations in the manufacturing process of thetouch panel 1, for example. The offset can be reduced to a minimum value(zero) by adjusting the capacitances CA3 and CA4 of the third and fourthelectrostatic capacitors C3 and C4 so that the capacitance CA3 is largerthan the capacitance CA4 by ΔC (CA3=C+ΔC, CA4=C).

When the capacitance CA1 of the first electrostatic capacitor C1 issmaller than the capacitance CA2 of the second electrostatic capacitorC2 by ΔC (CA1=C−ΔC, CA2=C), on the other hand, the capacitances CA3 andCA4 of the third and fourth electrostatic capacitors C3 and C4 areadjusted so that the capacitance CA3 is smaller than the capacitance CA4by ΔC (CA3=C−ΔC, CA4=C).

As an example structure of the third electrostatic capacitor C3 in thiscase, the third electrostatic capacitor C3 is formed to include melectrostatic capacitors C31-C3 m and m switches S31-S3 m, as shown inFIG. 5. It is preferable that capacitances CA31-CA3 m of theelectrostatic capacitors C31-C3 m are weighted so that the capacitanceCA3 of the third electrostatic capacitor C3 may be fine-adjusted. Forexample, when the capacitance CA31 of the electrostatic capacitor C31 isdenoted as C0, CA32=½ C0, CA33=¼ C0, CA34=⅛ C0, . . . CA3 m=½^(m−1) C0.Each of the switches S31-S3 m is turned on and off by corresponding eachof m-bits of adjustment signals from the calibration circuit 19. Thesame applies to the fourth electrostatic capacitor C4.

With the structure described above, the capacitances CA3 and CA4 of thethird and fourth electrostatic capacitors C3 and C4 can be adjusted bythe corresponding 2m-bits of digital adjustment signals from thecalibration circuit 19. The calibration circuit 19 can determine the2m-bits of adjustment signals with which the offset becomes a desiredvalue, which is preferably the minimum value, based on the outputvoltage Vout. The determined adjustment signals are written into andretained in an electrically writable/erasable non-volatile memory suchas the EEPROM 20.

Then, the adjustment signals written into and retained in the EEPROM 20are read-out from the EEPROM 20 when a power supply to the signalprocessing circuits 2X and 2Y is turned on. The calibration circuit 19adjusts the capacitances CA3 and CA4 of the third and fourthelectrostatic capacitors C3 and C4 based on the adjustment signalsread-out from the EEPROM 20.

As for the second sensor circuit of the single input type (Refer to FIG.4.), on the other hand, it is preferable that the capacitance CA1 of thefirst electrostatic capacitor C1 and the capacitance CA3 of the thirdelectrostatic capacitor C3 are equal to each other (CA1=CA3=C) in theinitial state. Note that this is for the case where the switch SW3 isturned off.

However, there is caused the offset in the output voltage Vout when thecapacitance CA1 of the first electrostatic capacitor C1 is larger thanthe capacitance CA3 of the third electrostatic capacitor C3 by ΔC(CA1=CA3+ΔC). The offset can be reduced by adjusting the capacitance CA3of the third capacitor C3 so that the capacitance CA3 approaches thecapacitance CA1 of the first electrostatic capacitor C1.

When the switch SW3 in the second sensor circuit of the single inputtype is turned on, the capacitance CA5 of the compound electrostaticcapacitor C5, which is composed of the third capacitor C3 and the fourthcapacitor C4, is adjusted in the same way as described above, in orderto minimize the offset. In this case, a variable range of thecapacitance CA5 can be extended because the electrostatic capacitor C5is the compound electrostatic capacitor, resulting in an advantage thatan adjustable range of the offset is extended.

When the touch panel is large in size, there is a case where the thirdelectrostatic capacitor C3 and the fourth electrostatic capacitor C4incorporated in the signal processing circuits 2X and 2Y alone could notprovide large enough capacitance because the capacitance CA1 of thefirst electrostatic capacitor C1 in the touch panel becomes extremelylarge. In that case, it is preferable that an external capacitor C6 isprovided outside the signal processing circuits 2X and 2Y so that it canbe connected in parallel with the third electrostatic capacitor C3 andthe fourth electrostatic capacitor C4 in the second sensor circuit ofthe single input type, as shown in FIG. 12.

In this case, each of the signal processing circuits 2X and 2Y isprovided with dedicated terminals 30 and 31 between which the externalcapacitor C6 is connected. A switch SW7 is connected between one end ofthe fourth electrostatic capacitor C4 and the dedicated terminal 30,while a switch SW8 is connected between another end of the fourthelectrostatic capacitor C4 and the dedicated terminal 31. With this, theexternal capacitor C6 can be added to complement the insufficientcapacitance by turning on the switches SW7 and SW8 with the switchingcontrol circuit 11.

The first through fourth input terminals CIN1-CIN4 in each of the signalprocessing circuits 2X and 2Y may be used instead of providing thededicated terminals 30 and 31. For example, the external capacitor C6 isconnected between the first input terminal CIN1 and the second inputterminal CIN2, as shown in FIG. 13. With this, an increase in the numberof the terminals can be avoided. In this case, although the externalcapacitor C6 connected between the first input terminal CIN1 and thesecond input terminal CIN2 is also connected to the first sensor circuitof the differential input type as shown in FIG. 14, it does not affectthe operation of the first sensor circuit.

The operation of the first sensor circuit of the differential input typedescribed above (Refer to FIG. 3.) is explained referring to FIGS. 6A,6B and 7. The alternating current drive signal SCDRV is a clock signalalternating between a high level (Vref) and a low level (groundvoltage=0 V). A voltage difference between an output voltage Vom fromthe inverting output terminal (−) of the differential amplifier 14 andan output voltage Vop from the non-inverting output terminal (+) of thedifferential amplifier 14 is the output voltage Vout (=Vop−Vom).

The first sensor circuit has a charge accumulation mode and a chargetransfer mode that alternate between each other.

First, when it is in the charge accumulation mode that is shown in FIG.6A, Vref is applied to the first and second electrostatic capacitors C1and C2. Also, the ground voltage (0 V) is applied to the third andfourth electrostatic capacitors C3 and C4.

The switches SW5 and SW6 are turned on. With this, the inverting outputterminal (−) and the non-inverting input terminal (+) of thedifferential amplifier 14 are short-circuited, while the non-invertingoutput terminal (+) and the inverting input terminal (−) areshort-circuited. As a result, a voltage at the node N1 (node of thewiring connected to the inverting input terminal (−)), a voltage at thenode N2 (node of the wiring connected to the non-inverting inputterminal (+)), a voltage at the inverting output terminal (−) and avoltage at the non-inverting output terminal (+) are all set to ½ Vref.A common mode voltage of the differential amplifier 14 in this case is ½Vref.

Next, when the first sensor circuit is in the charge transfer mode thatis shown in FIG. 6B, the ground voltage (0 V) is applied to the firstand second electrostatic capacitors C1 and C2, to the contrary of thecase in the charge accumulation mode. Also, Vref is applied to the thirdand fourth electrostatic capacitors C3 and C4. The switches SW5 and SW6are turned off.

The capacitances CA1, CA2, CA3 and CA4 of the electrostatic capacitorsC1, C2, C3 and C4 are equal to each other in the initial state(CA1=CA2=CA3=CA4). A difference between the capacitance CA1 and CA2 whenthe finger of the operator approaches the touch pad is represented by ΔC(CA1−CA2=ΔC). In this case, CA1=C+½ ΔC, and CA2=C−½ ΔC.

When in the charge accumulation mode shown in FIG. 6A, an amount ofelectric charges at the node N1 is represented by the followingequation:

$\begin{matrix}{{{Amount}\mspace{14mu}{of}\mspace{14mu}{Electric}\mspace{14mu}{Charges}\mspace{14mu}{at}\mspace{14mu} N\; 1} = {{( {C - {\frac{1}{2}\Delta\; C}} ) \cdot ( {{- \frac{1}{2}}{Vref}} )} + {C \cdot ( {\frac{1}{2}{Vref}} )} + {{CAf} \cdot 0}}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

where (C−½ΔC)·(−½ Vref) represents an amount of electric charges storedin C2, C·(½ Vref) represents an amount of electric charges stored in C4,and CAf·0 (=0) represents an amount of electric charges stored in Cf.

When in the charge transfer mode shown in FIG. 6B, an amount of electriccharges at the node N1 is represented by the following equation:

$\begin{matrix}{{{Amount}\mspace{14mu}{of}\mspace{14mu}{Electric}\mspace{14mu}{Charges}\mspace{14mu}{at}\mspace{14mu} N\; 1} = {{( {C - {\frac{1}{2}\Delta\; C}} ) \cdot ( {\frac{1}{2}{Vref}} )} + {C \cdot ( {{- \frac{1}{2}}{Vref}} )} + {{CAf} \cdot ( {{Vop} - {\frac{1}{2}{Vref}}} )}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

where (C−½ ΔC)·(½ Vref) represents an amount of electric charges storedin C2, C·(−½ Vref) represents an amount of electric charges stored in C4and CAf·(Vop−½ Vref) represents an amount of electric charges stored inCf.

[Equation 1]=[Equation 2] holds, since the amount of electric charges atN1 in the charge accumulation mode is equal to the amount of electriccharges at N1 in the charge transfer mode according to the law ofconservation of electric charge.

Following equation is derived by solving [Equation 1]=[Equation 2] forVop:

$\begin{matrix}{{Vop} = {{( {1 + \frac{\Delta\; C}{CAf}} ) \cdot \frac{1}{2}}{Vref}}} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

Similarly, following equation is derived by solving for Vom an equationobtained by applying the law of conservation of electric charge to anamount of electric charges at the node N2 in the charge accumulationmode and an amount of electric charges at the node N2 in the chargetransfer mode:

$\begin{matrix}{{Vom} = {{( {1 - \frac{\Delta\; C}{CAf}} ) \cdot \frac{1}{2}}{Vref}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

Vout is obtained from [Equation 3] and [Equation 4]:

$\begin{matrix}{{Vout} = {{{Vop} - {Vom}} = {\frac{\Delta\; C}{CAf} \cdot {Vref}}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

It is understood that the output voltage Vout of the first sensorcircuit of the differential input type varies in proportion to thedifference ΔC between the capacitance CA1 of the first electrostaticcapacitor C1 and the capacitance CA2 of the second electrostaticcapacitor C2.

It is assumed in the above calculations that CA1=CA2=CA3=CA4. When thereis a difference between CA1 and CA2 in the initial state, the offset inthe output voltage Vout can be made to a predetermined value or to theminimum value by adjusting CA3 and CA4 using the calibration circuit 19or the like as described above so that there is the same amount ofdifference between CA3 and CA4.

Next, characteristics of the output voltage Vout of the first sensorcircuit used in the touch sensor are explained referring to Table 2 andFIG. 7. As described above, the selection circuit 10 has the first phasein which it selects the signals from the first input terminal CIN1 andthe second input terminal CIN2 and the second phase in which it selectsthe signals from the third input terminal CIN3 and the fourth inputterminal CIN4.

The output voltage Vout of the first sensor circuit in the first phaseis represented as V1, while the output voltage Vout of the second sensorcircuit in the second phase is represented as V2. In this case, theoutput voltage V1 is proportional to the difference between thecapacitance of the capacitor formed between the Y sense line YL1 and theY drive line DRYL and the capacitance of the capacitor formed betweenthe Y sense line YL3 and the Y drive line DRYL.

Also, the output voltage V2 is proportional to the difference between acapacitance of a capacitor formed between the Y sense line YL2 and the Ydrive line DRYL and a capacitance of a capacitor formed between the Ysense line YL4 and the Y drive line DRYL. Then, the finger of theoperator or the like makes a single-touch on the touch panel 1 in arange between the Y sense line YL1 and the Y sense line YL4.

TABLE 2 Mode First Phase Second Phase Line V1 V2 YL1 +1 0 YL2 0 +1 YL3−1 0 YL4 0 −1

First, when the finger of the operator or the like touches the Y senseline YL1, the first output voltage V1 in the first phase becomes apositive (+) value. This is because the capacitance of the capacitorformed between the Y sense line YL1 and the Y drive line DRYL becomeslarger than the capacitance of the capacitor formed between the Y senseline YL3 and the Y drive line DRYL. And the second output voltage V2 inthe second phase becomes 0 V. This is because no change is caused in thecapacitance related to the Y sense line YL2 or YL4 since the finger ofthe operator or the like touches only on the Y sense line YL1.

Next, when the finger of the operator or the like touches the Y senseline YL2, the first output voltage V1 in the first phase becomes 0 V.This is because no change is caused in the capacitance related to the Ysense line YL1 or YL3. On the other hand, the second output voltage V2in the second phase becomes a positive (+) value. This is because thecapacitance of the capacitor formed between the Y sense line YL2 and theY drive line DRYL becomes larger than the capacitance of the capacitorformed between the Y sense line YL4 and the Y drive line DRYL.

Next, when the finger of the operator or the like touches the Y senseline YL3, the first output voltage V1 in the first phase becomes anegative (−) value. This is because the capacitance of the capacitorformed between the Y sense line YL3 and the Y drive line DRYL becomeslarger than the capacitance of the capacitor formed between the Y senseline YL1 and the Y drive line DRYL. On the other hand, the second outputvoltage V2 in the second phase becomes 0 V. This is because no change iscaused in the capacitance related to the Y sense line YL2 or YL4 sincethe finger of the operator or the like touches only on the Y sense lineYL3.

Finally, when the finger of the operator or the like touches the Y senseline YL4, the first output voltage V1 in the first phase becomes 0 V.This is because no change is caused in the capacitance related to the Ysense line YL1 or YL3. On the other hand, the second output voltage V2in the second phase becomes a negative (−) value. This is because thecapacitance of the capacitor formed between the Y sense line YL4 and theY drive line DRYL becomes larger than the capacitance of the capacitorformed between the Y sense line YL2 and the Y drive line DRYL. Maximumabsolute values of the first and second output voltages V1 and V2 arenormalized to “1” in Table 2 and in FIG. 7.

Note that the explanation above is based on the dielectric model inwhich the finger of the operator or the like is regarded as a dielectricmaterial and the capacitance of the capacitor related to the sense lineincreases when the finger of the operator approaches the sense line.Instead, when it is based on an electric field shielding model in whichthe finger of the operator or the like is regarded as a groundedconductor, the capacitance of the capacitor related to the sense linedecreases when the finger of the operator approaches the sense line.

FIG. 7 shows that the first and second output voltages V1 and V2 varycontinuously in accordance with the change in the touch position. Thatis, making a point on the Y sense line YL1 as an origin in FIG. 7 andmaking the X coordinate axis in FIG. 1 as a horizontal axis in FIG. 7,the first output voltage V1 is approximated by V1=cos X, and the secondoutput voltage V2 is approximated by V2=sin X. Therefore, it is possibleto detect the touch position (X coordinate) based on the first andsecond output voltages V1 and V2.

To show an example, since an equation V2/V1=tan X holds, the Xcoordinate of the touch position can be obtained using an equationX=arctan (V2/V1) and polarities (+, −) of the first and second outputvoltages V1 and V2. Here, arctan is an inverse function of tan. In thiscase, the first and second output voltages V1 and V2 are converted intodigital values with the A/D converter 17 and transmitted to themicrocomputer 3 through the I²C bus interface circuit 18 as describedabove. The X coordinate of the touch position can be obtained byperforming the calculation described above with the microcomputer 3.

Similarly, the Y coordinate of the touch position on the X sense linesXL1-XL4 can be detected based on the first and second output voltages V1and V2 through the operations of the signal processing circuit 2X. The Xand Y coordinates of the touch position can be obtained throughtime-series operations of the signal processing circuits 2X and 2Y asshown in FIG. 8, for example.

In particular, the first sensor circuit of the differential input typehas advantages of high sensitivity and high noise tolerance. However,the multi-touch can be not detected with the first sensor circuit insome cases. An example is a case where the finger of the operatortouches the Y sense lines YL1 and YL3 simultaneously. Since there is nodifference between the capacitance related to the Y sense line YL1 andthe capacitance related to the Y sense line YL3 in this case, the firstoutput voltage V1 becomes zero which makes no difference from theinitial state.

Next, the operation of the second sensor circuit of the single inputtype described above (Refer to FIG. 4.) is explained referring to FIGS.9A and 9B. The alternating current drive signal SCDRV is the clocksignal alternating between the high level (Vref) and the low level(ground voltage=0 V). A voltage difference between an output voltage Vomfrom the inverting output terminal (−) of the differential amplifier 14and an output voltage Vop from the non-inverting output terminal (+) ofthe differential amplifier 14 is the output voltage Vout (=Vop−Vom).

The switch SW3 is turned on. The third electrostatic capacitor C3 andthe fourth electrostatic capacitor C4 are connected in parallel to eachother and connected in series with the first electrostatic capacitor C1.The capacitance CA5 of the compound electrostatic capacitor C5 composedof the third electrostatic capacitor C3 and the fourth electrostaticcapacitor C4 is a sum of the capacitance CA3 and the capacitance CA4(CA5=CA3+CA4).

The second sensor circuit has the charge accumulation mode and thecharge transfer mode that alternate between each other.

First, when it is in the charge accumulation mode that is shown in FIG.9A, Vref is applied to the first electrostatic capacitor C1. And theground voltage (0 V) is applied to the compound electrostatic capacitorC5. The switches SW5 and SW6 are turned on. With this, the invertingoutput terminal (−) and the non-inverting input terminal (+) of thedifferential amplifier 14 are short-circuited, while the non-invertingoutput terminal (+) and the inverting input terminal (−) areshort-circuited. As a result, a voltage at the node N1 (node of thewiring connected to the inverting input terminal (−)), a voltage at thenode N2 (node of the wiring connected to the non-inverting inputterminal (+)), a voltage at the inverting output terminal (−) and avoltage at the non-inverting output terminal (+) are all set to ½ Vref.

Next, when the second sensor circuit is in the charge transfer mode thatis shown in FIG. 9B, the ground voltage (0 V) is applied to the firstelectrostatic capacitor C1, to the contrary of the case in the chargeaccumulation mode. And Vref is applied to the compound electrostaticcapacitor C5. The switches SW5 and SW6 are turned off.

The capacitances are set to be CA1=CA5=C in the initial state. Thecapacitance CA1 of the first electrostatic capacitor C1 is varied by ΔCwith a touch of the finger of the operator or the like. That is,CA1=C+ΔC and CA5=C.

When in the charge accumulation mode shown in FIG. 9A, an amount ofelectric charges at the node N2 is represented by the followingequation:

$\begin{matrix}{{{Amount}\mspace{14mu}{of}\mspace{14mu}{Electric}\mspace{14mu}{Charges}\mspace{14mu}{at}\mspace{14mu} N\; 2} = {{( {C + {\Delta\; C}} ) \cdot ( {{- \frac{1}{2}}{Vref}} )} + {C \cdot ( {\frac{1}{2}{Vref}} )} + {{CAf} \cdot 0}}} & \lbrack {{Equation}\mspace{14mu} 6} \rbrack\end{matrix}$

When in the charge transfer mode shown in FIG. 9B, an amount of electriccharges at the node N2 is represented by the following equation:

$\begin{matrix}{{{Amount}\mspace{14mu}{of}\mspace{14mu}{Electric}\mspace{14mu}{Charges}\mspace{14mu}{at}\mspace{14mu} N\; 2} = {{( {C + {\Delta\; C}} ) \cdot ( {\frac{1}{2}{Vref}} )} + {C \cdot ( {{- \frac{1}{2}}{Vref}} )} + {{CAf} \cdot ( {{Vom} - {\frac{1}{2}{Vref}}} )}}} & \lbrack {{Equation}\mspace{14mu} 7} \rbrack\end{matrix}$

[Equation 6]=[Equation 7] holds, since the amount of electric charges atN2 in the charge accumulation mode is equal to the amount of electriccharges at N2 in the charge transfer mode according to the law ofconservation of electric charge.

Following equation is derived by solving [Equation 6]=[Equation 7] forVom:

$\begin{matrix}{{Vom} = {( {\frac{1}{2} - \frac{\Delta\; C}{CAf}} ) \cdot {Vref}}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Similarly, following equation is derived by solving for Vop an equationobtained by applying the law of conservation of electric charge to anamount of electric charges at the node N1 in the charge accumulationmode and an amount of electric charges at the node N1 in the chargetransfer mode:

$\begin{matrix}{{Vop} = {\frac{1}{2}{Vref}}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$

Vout is obtained from [Equation 8] and [Equation 9]:

$\begin{matrix}{{Vout} = {{{Vop} - {Vom}} = {\frac{\Delta\; C}{CAf} \cdot {Vref}}}} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

It is understood that the output voltage Vout of the second sensorcircuit of the single input type varies in proportion to the differenceΔC between the capacitance CA1 of the first electrostatic capacitor C1and the capacitance CA5 of the compound electrostatic capacitor C5.

It is assumed in the above calculations that CA1=CA5=C in the initialstate. When there is a difference between CA1 and CA5 in the initialstate, the capacitance CA5 can be adjusted using the calibration circuit19 or the like as described above so that the offset in the outputvoltage Vout becomes a predetermined value or the minimum value.

Next, characteristics of the output voltage Vout of the second sensorcircuit used in the touch sensor are explained. The selection circuit 10sequentially selects each of the signals from the first input terminalCIN1, the third input terminal CIN3, the second input terminal CIN2 andthe fourth input terminal CIN4 one after another, as described above.For example, each of the Y sense lines YL1, YL2, YL3 and YL4 shown inFIG. 1 is sequentially selected and connected to the second sensorcircuit.

Therefore, the second sensor circuit outputs the output voltage Voutthat is proportional to a change in a capacitance of a capacitor formedbetween the Y drive line DRYL and each of the Y sense lines YL1-YL4.Thus, it is possible to detect the touch position based on the outputvoltage Vout of the second sensor circuit. For example, in the casewhere the finger of the operator touches the Y sense line YL1, theoutput voltage Vout becomes large when the Y sense line YL1 is selected.

Unlike with the first sensor circuit of the differential input type, themulti-touch can be stably detected with the second sensor circuit of thesingle input type, because it selects each of the Y sense lines YL1,YL2, YL3 and YL4 one at a time to detect the change in the capacitance.

Therefore, it is possible to switch between the first and second sensorcircuits so that the first sensor circuit of the differential input typeis normally put in operation using the single-touch to take theadvantages of high sensitivity and high noise tolerance and the secondsensor circuit of the single input type is put in operation instead whenthe multi-touch is desirable.

Switching between the differential input mode and the single input modeis performed by the switching control circuit 11 as described above. Theswitching control circuit 11 may be structured so as to execute the modeswitching operation at receipt of an external command, for example acommand transferred from the microcomputer 3 through the serial clockline 4 and the serial data line 5.

Or, the switching control circuit 11 may be structured so as toautomatically switch between the differential input mode and the singleinput mode based on the result of the detection by the sensor. Forexample, when the finger of the operator or the like is relativelyremote from the touch panel 1, the first sensor circuit of thedifferential input type is put in operation because high sensitivitysensing is required. Then, when the output voltage Vout of the firstsensor circuit exceeds a predetermined threshold voltage, switching tothe single input mode is executed as the finger of the operator or thelike is judged to have approached within a predetermined distance fromthe touch panel 1 or touched directly to the touch panel 1.

Although the X drive line DRXL and the Y drive line DRYL are providedand the alternating current drive signal SCDRV is supplied from thedrive terminals CDRV in the signal processing circuits 2X and 2Y to theX drive line DRXL and the Y drive line DRYL in the structure shown inFIG. 1, it is also possible to utilize the X sense lines XL1-XL4 and Ysense lines YL1-YL4 as the drive lines.

In this case, each of the X sense lines XL1-XL4 is provided with thealternating current drive signal CDRV from the signal processing circuit2X when the signal processing circuit 2Y is operating as the touchsensor. On the other hand, each of the Y sense lines YL1-YL4 is providedwith the alternating current drive signal CDRV from the signalprocessing circuit 2Y when the signal processing circuit 2X is operatingas the touch sensor. With the structure described above, the X driveline DRXL and the Y drive line DRYL and the like are no longer needed.Also, the signal processing circuits 2X and 2Y can be implemented with asingle chip.

Different touching methods, that are the single-touch and themulti-touch, can be implemented since switching between the differentialinput mode and the single input mode is possible with the signalprocessing circuit of the electrostatic capacity type touch panelaccording to the embodiment of this invention. For example, thesingle-touch is normally used to take the advantages of high sensitivityand high noise tolerance available with the differential input mode, andthe input mode is switched to the single input mode when the multi-touchis desirable.

Also, since the first and second variable capacitors for calibration areconnected in parallel to each other when the second sensor circuit is inoperation with the signal processing circuit of the electrostaticcapacity type touch panel according to the embodiment of this invention,a variable range of the capacitance of the variable capacitor can beextended to extend the adjustable range of the offset. As a result, itis made possible to adjust the offset even in a large size touch panel.

What is claimed is:
 1. A signal processing circuit of an electrostaticcapacity type touch panel comprising a plurality of sense linesincluding a first and second sense lines and a drive line being appliedan alternating current drive signal, comprising: a first sensor circuitof a differential input type selecting the first and second sense linesout of the plurality of sense lines and detecting a difference between acapacitance of a first electrostatic capacitor formed between the firstsense line and the drive line and a capacitance of a secondelectrostatic capacitor formed between the second sense line and thedrive line; a second sensor circuit of a single input type selecting thefirst sense line out of the plurality of sense lines and detecting achange in the capacitance of the first electrostatic capacitor; firstand second variable capacitors for calibration adjusting an offset in anoutput voltage of the first or second sensor circuit; and a switchingcontrol circuit controlling the first and second sensor circuits so thatone of the first and second sensor circuits is put in operation andcontrolling the first and second variable capacitors for calibration sothat the first and second variable capacitors are connected in parallelto each other when the second sensor circuit is put in operation,wherein the switching control circuit connects the first variablecapacitor in series with the first electrostatic capacitor and connectsthe second variable capacitor in series with the second electrostaticcapacitor when the first sensor circuit is put in operation, andconnects the parallel-connected first and second variable capacitors inseries with the first electrostatic capacitor when the second sensorcircuit is put in operation.
 2. The signal processing circuit of claim1, further comprising an external capacitor connected in parallel withthe variable capacitors for calibration.
 3. The signal processingcircuit of claim 2, further comprising a pair of terminals to which apair of signals from the electrostatic capacity type touch panel isinputted, wherein the external capacitor is connected to the pair ofterminals.
 4. The signal processing circuit of claim 1, furthercomprising a calibration circuit outputting an adjustment signal toadjust a capacitance of the first variable capacitor or the secondvariable capacitor based on the output voltage of the first or secondsensor circuit.
 5. The signal processing circuit of claim 4, furthercomprising a non-volatile memory retaining the adjustment signal.